The processing of animal hides to produce leather is an ancient art, and today it is a very mature industry. Excellent references to the chemistry of leather manufacture by McLauglin, G. D., et al, The Chemistry of Leather Manufacture, Reinhold Publishing Corp, N.Y. (1945), and collagen reactivity by Gustavson, K. H., The Chemistry and Reactivity of Collagen, Academic Press Inc., N.Y. (1956), date from the 1940's and 1950's, and are still basic descriptions of the art practiced today. The name "collagen" is derived from the Greek word for glue, as is the term "colloid" which means "gluelike" in Greek.
Skin is composed of four distinct layers, which are, proceeding from outside-in: (1) a thin outer layer of epithelium termed the "epidermis", which is rich in the protein keratin, not collagen; (2) a dense collagen-rich layer, termed the "dermal" or "grain" layer, also called in the older literature the "thermostat" layer; (3) a thicker layer of less-dense, collagen-rich connective tissues, termed the "corium" layer; and (4) an inner layer of "subcutaneous tissue", known to the tanner as "flesh", by which the skin is attached to the underlying tissue.
Although hides may merely be "cured" in salt and/or other biocidal solutions to stop microbial degradation, many hides that are intended for use in leather manufacture are "limed", that is, soaked in a saturated solution of hydrated lime (calcium hydroxide) and water. The liming process initiates the loosening of the epidermis and the subcutaneous layer, and is the first step in the dehairing process. After liming is complete, the hair, epidermis, and any residual flesh, fat and surface muscles are removed by mechanical scraping, and the dermal layer is mechanically cut, along with enough of the corium layer to give the final leather its required thickness, from the remaining inner corium layer.
In leather-making the primary interest is on the dense collagen-rich dermal layer, which is about 25% of the thickness of the corium layer. During the process of leather-making, the dermal tissue receives separate chemical and tanning treatments to stabilize the collagen structure.
The residual portion of the corium layer that is separated from the dermal layer is termed the "limed split" and is a by-product waste of the leather manufacturing process. It is these limed splits that become, for example, the collagen-rich feedstock for sausage casing production, and that have been used as the source of collagen for the examples herein.
During the liming process, the skin imbibes and binds water, and becomes highly swollen; in the process it acquires a very alkaline pH of about 12.5. The chemistry of the liming process is quite well understood. Prior to further leather processing, and in the collagen production process considered here, the skins must be "delimed" by soaking in acid or salt solutions.
Four U.S. Pat. Nos. (4,140,537, 4,233,360, 4,488,911, 4,655,980) all assigned to Collagen Corporation, describe enzymatic methods, including pepsin hydrolysis, for solubilizing collagen to produce a "non-immunogenic" soluble collagen, which is then converted to other forms for use as medical implants. In these patents, the initial soluble product is relatively low (for collagen) molecular weight aggregates (about 300,000 daltons); the objective is to remove all of the "telopeptides" which are found at the end of these chains. Higher molecular weight aggregates would not have the telopeptides completely removed, and would be more "immunogenic" by their standards.
A 1970 U.S. Pat. No. (3,532,593) describes a method for making collagen for use in papermaking. It describes a mechanical method for isolating preexisting gelled collagen fibers, not an enzymatic method for solubilizing the collagen as in the present invention. This patent describes a method for adjusting the pH of mechanically gelled collagen to promote flotation of fat and for skimming the floating fat from the collagen. The patent also refers to the partial "gelatinizing" of the collagen by heating to improve the bonding properties of the additive, although the primary objective is to produce a fibrous product.